Many recently developed composite materials, which typically consist of bonded layers of dissimilar materials, such as graphite composites including graphite/bismaleimide, graphite/phenolitic, graphite/polyamide, and other composites, have properties that cause them to be difficult to machine by conventional methods. In part, these difficulties arise due to relatively slow cutting speeds of cutting tools, which generate heat that in turn causes separation (delamination) of the various layers of materials. Also, as the design of conventional cutting tools depends on relatively high instantaneous pressures to advance cutting edges of the tool into the composite, chipping of the composite frequently results. Further, many of these composites possess highly abrasive characteristics and tend to wear cutting edges of conventional cutting tool bits at inordinately high rates.
One attempt to circumvent these problems has involved the use of chemical etching wherein regions of a composite component that are to be unaffected by the etching process are coated with a "resist" material, with areas to be removed being exposed to a corrosive agent.
Problems with the etching process are that it is time-consuming and difficult, if not impossible, to precisely control removal of material from the component, making the process unsuitable for precision work. Additionally, there exists a risk of contamination to the environment and hazards to workers due to toxicity and corrosiveness of the materials used in the etching process.
Manually operated cutting tools, such as die grinders, hand-held drills, and the like have also been tried and have been found to be unsatisfactory because they are labor intensive and not productive of precise tolerances required by aeronautical and aerospace applications. Additionally, as one cannot hold or move a motor driven, hand-held tool as precisely as a machine, the life of tool bits constructed of hard, brittle materials, such as carbide, is greatly reduced.
In addition to these problems related to machining of composite materials, advanced hardened metallic alloys such as Inconel, Waspaloy, NASA-23, 18-8 stainless steel, alloys of aluminum and lithium, 22-19 aluminum, Incaloy 903, titanium alloys, and other recently developed metallic alloys, present a wide range of problems when they are attempted to be machined. Here, characteristics of these materials that make them particularly applicable to aeronautical and aerospace applications, which include low mass, high melting temperature, hardness and toughness, etc, frequently results in the workpiece being chipped or otherwise roughened by spalling, galling, heat working, and other deleterious processes caused by relatively low speed of the tool bits during the machining process. Because of these deleterious processes, further grinding and polishing is required to finish the machined article. Additionally, as each cutting edge of a machine tool can only remove a tiny amount of the hardened material, typically only 0.001 to 0.002 inch per pass, these machining processes are time-consuming because of slow feed rates and expensive due to breakage and wear of tool bits, some of which may cost hundreds or thousands of dollars.
In these conventional machining processes, as far as is known by the applicant, an electric motor powers a spindle to which in turn is mounted cutting tools or bits used in the machining process. Where the motor is powered by alternating current, only limited speed control of the motor is possible, generally due to multiple, selectively powered sets of windings in the motor, with motor speed dependent on the particular winding that is energized.
In the instance of direct current motors, which are typically servo motors, rotational speed of the motor is controlled up to approximately 3,600 rpm by varying voltage applied to the motor. However, where an electrical motor is coupled to a spindle, sonic and harmonic vibrations from the motor, in addition to the incremental power surges that occur during operation as each respective winding of the motor is energized, are transmitted to the tool bit. It is believed that these surges and vibrations contribute to "dwell," a phenomena characterized by cutting surfaces of the cutting tool or bit momentarily hanging or digging into the surface of the workpiece being cut, which may result in dangerous explosive destruction of the fragile, brittle cutting tool or bits, and which certainly causes excess wear thereto. Additionally, the power surges and harmonic and sonic vibrations cause "chatter," which in turn causes chipping and high wear of the tool bits. Further, servo motors require control circuitry, making them expensive, some of which approach approximately $80,000 in cost. Also, these bulky, heavy electrical motors do not lend themselves to be mounted to robotic manipulators or movable components of computer numeric control (CNC) machine tools, which are capable of supporting and moving only limited weight.
In addition to problems related to the described power surges and harmonic and sonic vibrations associated with electrically powered spindles, conventional cutting bits are typically mounted at positive angles (the cutting edge angled toward the direction of rotation) with respect to the workpiece so as to dig or shave material therefrom. This generates forces which tend to pull the cutting bits into the workpiece, which, in conjunction with any side or axial play or looseness in the spindle, contributes to the problems of dwell and chatter. As such, cutting bits are constructed having small cutting surfaces and are generally configured to limit the amount of material that is removed by a cutting bit in a single pass or rotation. As a result, pressure and relatively low speed of the cutting tool is often required to advance the cutting bits into the workpiece, causing friction that generates large amounts of heat in the surface of the workpiece being cut, the resultant shavings, and the cutting bits. This heat, which otherwise would destroy the temper in treated alloys and the cutting bits, or cause warpage of the workpiece, is conventionally carried away by a coolant applied directly to the cutting or machining operation. As such, some machine tools must be equipped with provisions for pumping, applying, filtering and recycling such coolants, adding to their costs and to costs of the machining operation, as these coolants must be periodically replaced.
In some instances, such as tool post grinders, an electrical motor is coupled to a spindle via a belt drive and, through a step-up pulley system, is capable of driving the spindle at speeds of up to approximately 12,000 rpm. In this instance, some of the harmonic and sonic vibrations from the electrical motor are isolated by separation of the rotational member of the motor from the rotating spindle by the drive belt. However, since the motor is typically fixed to the same mounting member as the spindle, some motor vibrations are transmitted to the spindle, resulting in excess wear of the tool and less than optimum machine work as described. Further, with a belt drive, there is greater possibility of encountering dwell due to flexure of the belt during operation, which causes an elastic snapping or whipping motion of the spindle and tool head. Further yet, these motors and spindles are expensive due to precision construction and are relatively expensive to operate due to the necessity of frequent belt replacement.
The applicant has developed a portable, hand-held apparatus for cutting ends of boiler tubes at relatively precise angles. In this reference, which is applicant's U.S. Pat. No. 4,819,526, a portable pushcart supports a motor coupled to a hydraulic pump and reservoir of hydraulic fluid, the pump providing a source of hydraulic fluid under pressure. A high-pressure line and return line are coupled from the pump to a hand-held tool which includes as a drive element a hydraulic motor, which develops about 45 inch-pounds of torque and has an output shaft capable of being driven at speeds from approximately 3,000 rpm to approximately 12,000 RPM, depending on the particular hydraulic motor used. A cutting tool mounted to the motor is disclosed as having a pilot shaft for centering the tool in a boiler tube, which tool also is provided with conventional carbide cutters for cutting edges of the tube. The cutters are each angled with a negative back rake and are provided with means for limiting the cut of each cutter to a maximum cut of 0.002 inch of the boiler tube end per pass. Also disclosed in this reference is the advantage that, since the tool is a hand-held tool, the hydraulic motor will absorb impact forces and shock when the cutting means engages a tube, reducing the probability that the cutting edges of the carbide cutters will be chipped or broken as a worker manually engages the tool with the end of a boiler tube.
While this apparatus works well for cutting relatively precise angles on boiler tube ends, it is not adaptable for any other operation, such as surface cutting (milling), involving precise, high-speed machining operations of composite or esoteric alloyed metals such as those found in aeronautical and aerospace applications due to the fact that it is a portable, hand-held tool rotating from approximately 3,000 to 12,000 rpm and is constructed only for cutting the end of a tube at a selected angle. Further, since the maximum cut of each cutter is limited to 0.002 inch, feed rates of material to this tool would correspondingly be limited to about 72 inches per minute at the maximum disclosed rotational speeds of 12,000 rpm of the cutting tool. Additionally, there is no suggestion in the reference that the apparatus may be subjected to relatively high side (radial) loads, such as are required when high feed rates on the order of 200 to 600 inches per minute of material are encountered during surface cutting operations, the prior art device only being intended to receive axial loads as it is held by hand against the end of a boiler tube. Additionally, there is no provision for varying rotational speed of the motor and cutter or for holding rotational speed of the cutter constant.
Further yet, as the entire unit is portable and highly mobile, being mounted on a pushcart, the drive motor provides only enough power to cut boiler tube ends, which is not sufficient power for the high-speed machining operations contemplated by the instant invention.
Lastly, the pilot shaft of the hand-held tool, as described in the reference, could bind and cause the tool to abruptly cease rotation while engaged with a boiler tube, transferring the motor torque to hands and arms of the user, possibly causing injury thereto.
Accordingly, it is an object of this invention to provide a compact, lightweight, low-cost, hydraulically powered, high power, high-speed spindle and power system and methods for machining therewith, the apparatus being capable of withstanding relatively high side loads and being adaptable for utilizing a variety of newly-developed machine cutters of the hardest, most brittle materials. As an additional feature of the invention, the rotational speed of the spindle is selectable, with the selected speed being held constant irrespective of loading of the spindle.
As a further object of the invention, a method of machining is described featuring high feed rates, while generally confining heat generated by the machining process to the removed material.